Progress in microelectronics in recent years has resulted in a miniaturization of devices and integration of functionalities. This has led to an effective and economic application of integrated, decentral systems in automation technology. Thus, not only are measured values ascertained in sensors and actuators, but also the measured values are already preprocessed, linearized and even a self-diagnosis of the sensor or actuator implemented. Prerequisite for introduction of these decentral functionalities into a closed automation concept with “intelligent” sensors and actuators is an increased information- and data-exchange of these decentral units among themselves and with a control system. In automation technology, for this reason, many fieldbus systems have been proposed lately. These cover either company-specific application areas (e.g. BITBUS, CAN, MODBUS, RACKBUS) or are the subject of international standard (e.g. HART, PROFIBUS-PA, Foundation FIELDBUS, Ethernet). A large number of fieldbus systems currently used in industrial automation technology and process control technology fall, furthermore, still only under the heading “fieldbus”, or “fieldbus system”.
Usual field devices are grid-fed, four-conductor, measuring devices, and must, for such purpose, have at least two electrical wires leading to the device for energy supply. Furthermore, two more, signal conductors are needed, which transmit the signal carrying the measured value or other communication data signals between the decentral units and a control location. In general, this measurement signal or communications data signal is produced and forwarded according to a standard usual for such purposes, e.g. the current loop standard (involving an electrical current lying between 4 mA and 20 mA), a frequency standard, or a digital standard.
Moreover, in automation technology, it is also common to construct the field devices according to a so-called two-conductor technology and to connect them with one another, so that energy feeding and communications between the devices can occur via a single pair of lines, whereby the wiring effort and, consequently, the networking costs of networked, decentral automation systems can be decreased.
Such two-conductor field devices are treated in, among others, EP 0 883 097 B1, EP 0 895 209 B1, EP 1 158 274 A1. These two-conductor field devices produce as a measurement signal an output current, whose instantaneous values match as proportionally as possible a measuring transducer signal produced by means of a physical-to-electrical, measuring transducer. The two conductors serve both for supply of energy (under open- or closed-loop control), for which a direct voltage source is applied externally onto the two conductors, as well as also for transmission of the measurement signal.
A further aspect, which must be heeded in automation technology and especially in process automation technology, is that the field device must be able to operate in an explosion-endangered area, with intrinsic safety of the field device and fieldbus assured. Such intrinsically safe, low-reflection, fieldbus systems are known from DE 100 49 233 A1, DE 101 27 561 A1, DE 102 45 273 A1 and the CIS (continuous interruption supply) energy supply concept of the Physikalisch-Technische Bundesanstalt Braunschweig, Germany, which treat the problems of alternating voltage- and alternating current-signals on fieldbusses. Common to all these citations is that the energy transmission over the fieldbus is done alone by a high-frequency alternating signal and this fieldbus fulfills intrinsic safety by protective measures and/or is embodied to be low reflection by adaptations of the wave resistance.